The invention is more particularly directed to a trimming system for a boat drive of the above mentioned type, by means of which the pitch angle of the drive body with the output propeller shaft can be controlled from between one or a few degrees above the horizontal plane to about 8-10 degrees below the horizontal plane, or preferably about +/- 5-8 degrees from a normal pitch angle of 3-6 degrees.
A boat drive of this type is known from U.S. Pat. No. 4,645,463 (H M Arneson) which patent discloses a structure in which the drive body is formed with a ball over which the drive is connected to a part of the inboard engine, in that said ball is journalled in a ball carrier which is mounted at the stern of the boat, whereby the drive can be rotated universally, and which drive is formed with at least one hydraulic cylinder for trimming the drive up and down, and at least two further hydraulic cylinders for rotating the drive in the horizontal plane when turning the boat left or right.
A boat drive of this type having a water surface driving propeller is advantageous as compared with the so called Z-drive (inboard-outboard) type, and above all the drive is subjected to less flow losses and less power losses than boat drives having angle gears and transmission gear sets. Depending on the simple structure of the drive it is also cheaper, more effective and apt to less wear and has less sources of errors than many other types of boat propulsion drives or gears.
The apparatus known from U.S. Pat. No. 4,645,463 indeed involves the advantages of a drive being a straight, surface water driving propulsion drive, but it is disadvantageous in that the ball and the ball carrier are subjected to strong stresses; in that certain plays may appear in the steering means thereof; in that the hydraulic cylinders for the trimming (tilting) and for the steering operations need to be serviced and maintained, are subjected to wear and are sensitive to ruptures and leakage in the hydraulic conduits; and in that there is a need for long conduits and/or hoses from the propulsion drive at the outside of the boat to the maneuvre place inside the vessel or boat.
Normally surface driving propellers have a bad back driving capacity and this also in a disadvantage of the above mentioned apparatus.